You could try Indoor Gardening Supplies, if 49" x 20" is close enough. Else call them; they might be able to obtain one that fits your exact measurements.

dcleaver47,

What search terms are you using?

You have to remember that anything you buy on the internet has to be shipped and delivered. A 2' X 4' tray is something of a large box to ship. Some places can get a flood and drain tray to you in one piece and some may not have so much experience.
Often a large light weight box is shipped by what is called "dimensional weight" (the rule of thumb is to calculate the shipping weight at 30 pounds).

Another cost cosideration is whether or not the tray is drilled out and includes a set of feed/drain fittings. Some sites include fittings and drilling, some sites charge extra.

4 foot by 4 foot or 4 foot by 8 foot trays have to be delivered by truck. The packages are too large and cannot be shipped by mail, UPS, DHL, FEDex, etc. So, while you can always buy one of the large trays, it is undoubtedly more cost effective to buy more than one to ship at a time.

You might want to do a search for the "Bigfoot" system. This is a system which comes prepackaged in an easy and economical to ship box.

Look at this link. I have 4 of the Large ones they fit perfect on the wire grow shelfs and might work for you. They are very nice and made of very durible plastic.

Here is a link that might be useful: Boot Trays

I built my own years ago when I was first experimenting with growing under lights. I used a piece of plywood, some 2x2's and stapled water-resistant fabric over all.

Now that you mention it, maybe I'll go build another for the top of my new light rack.

You can get 4ft by 4ft plywood at Home Depot, and whomp up your own trays.